A method of forming a direct positive image is well known, where a direct positive photographic material having a previously non-fogged internal latent image-type silver halide emulsion is imagewise exposed and then subjected to surface-development after or during fogging.
The above-mentioned latent image-type silver halide photographic emulsion means a silver halide photographic emulsion of such a type that the silver halide grains therein have light-sensitive nuclei essentially in the inside thereof and a latent image is formed essentially in the inside of the grains by exposure.
Various techniques have been known in this technical field. For instance, U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577 and British Pat. Nos. 1,151,363, 1,150,553 and 1,011,062 illustrates essential techniques of the field.
Using the known methods, direct positive photographic materials having a relatively high sensitivity can be obtained.
The details of the mechanism of forming direct positive images are described, for example, in T. H. James, The Theory of the Photographic Process, Ed. 4, Chap. 7, pages 182 to 193 and U.S. Pat. No. 3,761,276.
In order to put a direct positive photographic material into practical use, the material must give a hard (high contrast) image, having a high Dmax and a low Dmin. In general, a direct positive photographic material has drawbacks in that the time-dependent variation of sensitivity during storage (or increase of sensitivity) is large and the aged material often gives an image having a lowered maximum sensitivity. Therefore, elimination of such drawbacks has been desired in preparing a direct positive photographic material.